PSNI Chief Constable Simon Byrne has warned he faces losing 300 officers because of cuts just as crime is set to rise with the lifting of coronavirus restrictions.
He told the Policing Board a £23m funding gap would lead to police officer numbers falling to 6,719 over the next 12 months.
Mr Byrne said this would equate to "10 police support units, TSG type of things that we use in protests, parades and disorder, or 10 major incident teams that could be dealing with serious organised crime, terrorism and murders".
He noted a recent increase in anti-social behaviour and warned a fall in crime as a result of the pandemic would soon be reversed as lockdown eases.
"We are concerned that as we go into April we will face increasing pressure and demand, but realistically with a gradually declining workforce," he said.
Mr Byrne said the PSNI was getting further away from a peak workforce of 7,500 a decade ago, despite a commitment to return to that number in the New Decade, New Approach agreement.
He added police had expressed their concern to Justice Minister Naomi Long and Secretary of State Brandon Lewis and had asked the Department of Justice and NIO for more funding.
The Chief Constable said the NIO bid was related to "additional pressures around policing the EU exit and protocol and actually making sure we've got police officers available in some of those communities that clearly at the minute are most affected".
"The real life choice we face is that at some point there will be less detectives, less people policing the roads or less people available to respond to protests and disorder," he said.
"Service will look and feel differently, we cannot carry on with the same level of service with 300 less people. It's inevitable that something will have to change."
Mr Byrne said reducing police numbers would also "impede our efforts to become a more inclusive and representative workforce".
"If we recruit less police officers and police staff from wherever, that will play out in terms of a conversation in a couple of years of who our sergeants, first grade supervisors and police staff are," he said.
"It's not just in terms of putting more officers on the streets."
He revealed police were continuing to be busy dealing with Covid-19-related issues, handing out £20,000 in fines after parties in the Holyland area of Belfast this week.
He said January had seen a peak in terms of demand for police, as officers dealt with 3,639 Covid-related incidents.
He said 1,080 fixed-penalty notices of £200 were issued, with the majority related to gatherings in private dwellings.
Police were now preparing for St Patrick's Day, the Easter holidays and the Twelfth of July.
"As the restrictions change our plea is that we're given good notice of that and also appropriate guidance if the regulations change," he said. And Mr Byrne expressed concern about a 13.6% rise in assaults on police officers, which he described as "utterly unacceptable".
He noted there had been a 22% rise of grievous bodily harm with intent, which he said was related to spitting on officers, and "particularly concerning in the Covid environment".
"This is not an acceptable way of treating loyal public servants; neither police officers nor police staff come to work every day expecting to be assaulted, and nor should they be," he added.